The present invention relates to a control apparatus for variably controlling a speed of an induction motor including a PWM inverter for converting a fed AC power supply to an alternating current having a desired frequency and a desired voltage and for feeding the converted alternating current to an induction motor so that the speed of the induction motor may be controlled variably without using any speed sensor. More specifically, the present invention relates to a control apparatus for variably controlling a speed of an induction motor that facilitates starting the PWM inverter with a high reliability or restarting the PWM inverter with a high reliability after the AC power supply has recovered from a momentary service interruption. Hereinafter, the control apparatus for variably controlling the speed of the induction motor will be sometimes referred to simply as a "speed-control apparatus".
Japanese Unexamined Laid Open Patent Publication (KOKAI) No. H08-130882 discloses a method of controlling a speed of an induction motor.
FIG. 18 is a block diagram of the conventional speed-control apparatus of an induction motor disclosed in the above described patent application.
Referring now to FIG. 18, the reference numeral 1 designates an induction motor, 2 a PWM inverter, 3 a current detector and 10 a speed-control apparatus. The speed-control apparatus 10 includes a speed control system 11, a magnetic flux control system 12, a current control system 13, two-phase three-phase converting means 14 and three-phase two-phase converting means 15, which are constructed by the conventional techniques. For starting the PWM inverter 2 or for restarting the PWM inverter 2 after an AC power supply (not shown) has recovered from a momentary service interruption, the speed-control apparatus 10 also includes a signal generator 16, adders 17a and 17b, magnetic flux estimating means 18 and frequency calculating means 19.
In the speed-control apparatus 10 shown in FIG. 18, the fundamental equation of the induction motor 1 rotating at an angular frequency .omega..sub.2 is expressed on the stationary coordinate having .alpha.-axis and .beta.-axis by the following equation (1). EQU e.alpha.=s.multidot..PHI..alpha. EQU e.beta.=s.multidot..PHI..beta. EQU 0=-R.sub.2 i.alpha.+(s+1/T.sub.2).PHI..alpha.+.omega..sub.2 .PHI..beta. EQU 0=-R.sub.2 i.beta.+(s+1/T.sub.2).PHI..beta.-.omega..sub.2 .PHI..alpha.(1)
Here, e.alpha. and e.beta. are .alpha.-axis and .beta.-axis components of the induced voltage of the induction motor 1; i.alpha. and i.beta. are .alpha.-axis and .beta.-axis components of the primary current of the induction motor 1; .PHI..alpha. and .PHI..beta. are .alpha.-axis and .beta.-axis components of the magnetic flux of the induction motor 1; R.sub.2 is a conversion value on the primary side of the secondary resistance of the induction motor 1; T.sub.2 is a secondary time constant of the induction motor 1; and s is a differential operator for Laplace transformation.
The above equation (1) is described by the functional block diagram described in FIG. 19. From this figure, .PHI..alpha. and .PHI..beta. are expressed by the following equations (2) and (3), respectively. EQU .PHI..alpha.=[R.sub.2 (s+1/T.sub.2)i.alpha.-.omega..sub.2 R.sub.2 i.beta.].div.[(s+1/T.sub.2).sup.2 +(.omega..sub.2).sup.2 ](2) EQU .PHI..beta.=[R.sub.2 (s+1/T.sub.2)i.beta.+.omega..sub.2 R.sub.2 i.alpha.].div.[(s+1/T.sub.2).sup.2 +(.omega..sub.2).sup.2 ](3)
In the equations (2) and (3), the relation (1/T.sub.2).sup.2 &lt;&lt;(.omega..sub.2).sup.2 is held in the ordinary driving mode in that the PWM inverter 1 converts the fed AC power supply to an alternating current having a desired frequency and a desired voltage and controls the speed of the induction motor 1 variably. Therefore, the vibrating frequency of the magnetic flux of the induction motor 1 rotating at an angular frequency .omega..sub.2 is .omega..sub.2.
That is, by feeding a primary current containing a frequency component .omega..sub.2 to the induction motor 1 rotating at the angular frequency .omega..sub.2 by the signal generator 16 and the adders 17a and 17b, a rotating magnetic field with the frequency of .omega..sub.2 is generated in the induction motor 1.
In practice, the current detector 3 detects the primary current of the induction motor 1. The three-phase two-phase converting means 15 converts the detected current values i.sub.U and i.sub.W to i.alpha. and i.beta., i.e. the .alpha.- and .beta.-axis components on the stationary coordinate. The magnetic flux estimating means 18 estimates .PHI..alpha. and .PHI..beta. from the equations (2) and (3) by using the converted i.alpha. and i.beta., and the frequency calculating means 19 obtains the angular frequency .omega..sub.2 from the estimated .PHI..alpha. and .PHI..beta.. The obtained angular frequency .omega..sub.2 is used as a rotating speed data (f).
For starting the PWM inverter 2 or for restarting the PWM inverter 2 after the AC power supply has recovered from a momentary service interruption, the speed-control apparatus 10 of FIG. 18 makes the signal generator 16 work for a predetermined period of time at first. Then, the speed-control apparatus 10 starts the PWM inverter 2 based on the rotating speed data (f) obtained during the foregoing predetermined period of time to avoid overload of the PWM inverter 2 at its start.
As explained above, the conventional speed-control apparatus feeds a primary current containing a frequency component .omega..sub.2 using the signal generator 16 and the adders 17a and 17b. At this moment, the speed-control apparatus 10 obtains the rotating speed data (f) through the magnetic flux estimating means 18 and the frequency calculating means 19. Therefore, it is required for the current control system 13 to respond quickly. For preparing the required function of the current control system 13, it is necessary to employ an expensive high-speed microcomputer. For obtaining the rotating speed data (f) by the fast Fourier transform (FFT), it is necessary to conduct complex calculations.
In principle, the vibrating frequency of the magnetic flux of the induction motor is identical to the rotating angular frequency of the induction motor. Since the vibration amplitude of the magnetic flux decades with the secondary time constant (T.sub.2) of the induction motor, the vibration amplitude decades relatively faster especially when the induction motor is rotating at a low speed. Due to the relatively faster decade of the vibration amplitude, it is sometimes difficult to obtain the rotating speed data (f).
In view of the foregoing, it is an object of the invention to provide a control apparatus for variably controlling a speed of an induction motor, wherein the foregoing problems are obviated.
It is another object of the invention to provide a control apparatus for variably controlling a speed of an induction motor, wherein a cheap or economical microcomputer can be used.
It is a further object of the invention to provide a control apparatus for variably controlling a speed of an induction motor, wherein rotating speed data (f) of the induction motor can be obtained by simple operation even when the induction motor is rotating at a low speed.